Terrain following radars, threat information and stored terrain data bases are typically used on military aircraft to supply information to the aircraft pilot, or autopilot system when the autopilot system is in use, to permit or require the aircraft to fly along a path which approximates the contour of the terrain below, at a predetermined elevation above the terrain. When the aircraft is flying a mission in hostile territory, it has a greater chance of avoiding detection by the enemy by flying low over the terrain. Although the probability of detection is minimized by flying low, the danger of crashing into elevated terrain is increased, particularly when the terrain is rough. Terrain following radars are described in greater detail in U.S. Pat. Nos. 3,277,467; 3,397,397; 3,623,093; 3,657,719; 3,739,380; 3,795,909; 3,815,132; and 4,016,565.